The first route to receive #Intercity125 (aka #HST) trains was the #GreatWestern Mainline (GWR).

Constructed by #IsambardKindomBrunel and completed in 1841, the line's alignment and gradients (Between Paddington and Didcot 1 in 1320 - 0.075 %) are well suited to high speeds trains.

Whilst largely superceded now, HST's were quite comfortable for passengers but they could produce significant fumes and noise (similar to a jet engine) especially when powering up.

See:

https://youtu.be/C1ICcOu0NkM